10 REPRESENTATIONS OF ALGEBRAS OF CONTINUOUS FUNCTIONS 389 It is immediately verified that if U is a hermitian (resp. positive hermitian, resp. unitary) operator on a Hilbert space E, and if S is an isomorphism of E onto a Hilbert space E', then the operator SUS'1 on E' has the same property (since these properties involve only the Hilbert structure). Hence we may take T(u) = M^(u), and in this case the sufficiency of the conditions stated is clear. On the other hand, if for example there exists a measurable subset X of K such that ju(X) = a > 0 and J(u(x)) ^ j8 > 0 for all x e X, then we have </((T(w) - <pxl<Px)) = s(\ udp\^afl and therefore T(u) is not hermitian.,The other cases are dealt with similarly. (15.10.6) (i) The orthogonal projectors belonging to the algebra are the operators of the form T((px), where X is a universally measurable subset ofK. (ii) T(^C(K)) is a maximal commutative subalgebra of&(E). (iii) A closed vector subspace EofE is stable under T if and only if it is of the form T(<px)(E), where X is a universally measurable subset o/K. (i) By virtue of the characterization of orthogonal projectors on E (15.5.3.1) and by (15.10.5), it follows that T(u) is an orthogonal projector if and only if u is almost everywhere equal to a /(-measurable bounded real-valued function v such that v2 = v. Hence v = <px, where X is a /(-measurable subset of K, and the result follows. (ii) It is clearly enough to prove that a continuous operator Fe JS?(E) which commutes with T(u) for all ue ^C(K) is of the form T(v). If a is a totalizing vector for T, then for each universally measurable subset X of K we have *)V-a\a)\ = \(VT(q>d-a\T(q>d-a)\ iin<px)^ii2 = iim- [<pxdnata = \\v\\- because T(<px) is an orthogonal projector which commutes with V. The Lebesgue-Nikodymtheorem (13.15.5(cy)) therefore shows that uv.a}0 = h • n for some /x-integrable function h. Since also it follows from the inequality above that for every step function we^c(K), we conclude that the linear form w h-* I hw d\i defined on the vector space of these step functions extends bynd w are in ^C(K), tnen f°r